Coal with improved combustion properties

ABSTRACT

A method for improving the combustion properties of a coalcomprises treating said coal with a metal porphyrin. The invention also provides a coal having a metal porphyrin deposited thereon, and a method of producing heat, comprising combusting the coal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for improving the combustion properties of a coal, to a coal with improved combustion properties, and to a process for combustion of a coal with reduced emissions.

2. Description of the Prior Art

Incomplete combustion in coal fired furnaces results in carbon being retained in ash, and limits the efficiency of coal fired plant. The carbon in ash contributes to the overall ash emission, reduces the efficiency of electrostatic precipitators for ash removal, and makes the ash less easy to dispose of, for example as a component for cement.

Many coal-fired power plants, including Russian and Chinese plants, are using low-grade coals with low reactivity. The main challenges faced when firing these grades of coal are:

-   -   High carbon content in the fly ash: up to 15-20%;     -   Significant NOx emission.

The high content of unburned carbon in the fly ash results in the significant heat losses: up to 5% or even higher depending on the coal ash content.

The NOx concentration in effluent gases at air excess (α) of 1.4 is 700-900 mg/m³ (recalculated to NO₂) depending on the boiler power.

EP1498470 lists several methods for reducing carbon in ash from coal burning, including increasing the excess of air introduced with the fuel, or adding metals such as calcium and magnesium. These methods have undesirable effects, with increased air causing higher NOx emissions, and use of metals such as calcium and magnesium requiring large amounts, and causing fouling of the system. EP1498470 proposes the addition of 2-500 ppm of a manganese compound, preferably a manganese tricarbonyl compound.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a method for improving the combustion properties of a coal, the method comprising treating said coal with a metal porphyrin.

A second aspect of the invention provides a coal having a metal porphyrin deposited thereon.

We have found that the invention can provide improved carbon burnout, resulting in reduced carbon content in the ash. The activation energy for oxidation may also be reduced. NO_(x) formation in combustion is related to the excess of air over the stoichiometric requirement: more excess means higher NO_(x) and lower thermal efficiency. Improved rates of combustion/lower activation energy tends to reduce excess air requirement and lower NO_(x) production. The combustion chamber airflow is typically actively managed and can be altered to optimise combustion conditions to minimise carbon content in ash and minimise NO_(x).

The invention is of particular applicability to low-grade coals such as brown coal or bituminous coal.

The metal porphyrin of the present invention preferably contains a metal with two or more possible oxidation states. Examples include transition metals such as iron, cobalt or manganese.

The metal porphyrin additive may be put up in an aqueous solution and applied to the solid fuel by methods commonly known in the art, for example by spraying onto the solid fuel. Alternately the metal porphyrin is applied by sublimation and vapour deposition.

Porphyrins occur widely in nature, and they play very important roles in various biological processes. Synthetic porphyrins such as phthalocyanines have industrial uses, for example copper phthalocyanine is widely used as a cyan pigment. Porphyrins are fully aromatic systems, are capable of hosting a wide variety of metal atoms and have high thermal stability. Porphyrins can be modified, for example by sulphonation, to change their solubility in various media.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example only, with reference to the following drawings, in which:

FIGS. 1-3 are graphs of, respectively, TG, DTG and DTA results for a coal in accordance with an aspect of the invention and comparative samples;

FIGS. 4-6 are graphs showing linearised DTG data for, respectively, untreated brown coal, H₂SO₄-treated brown coal and Fe additive treated brown coat;

FIGS. 7 and 8 are graphs of DTA results for untreated brown coal and for brow coal treated, respectively with an iron-based additive in accordance with the invention and a cobalt-based additive in accordance with the invention;.

FIGS. 9-11 are graphs showing, respectively, TG, %sample weight loss, and DTG results for untreated brown coal and brown coal treated with Fe and Co additives in accordance with the invention.

DETAILED DESCRIPTION

Thermal analysis methods, such as thermogravimetry (TG), differential thermal analysis (DTA), and differential scanning calorimetry (DSC), have been employed extensively in investigations relating to coal utilization.

Thermogravimetry (TG) is widely used to investigate coal/char reactivity. It is well documented that the reactivity depends on coal rank, maceral composition and/or charring temperature. The coal combustion reactivity was measured by TG, in general, under two conditions (i) isothermal, at a constant temperature, and (ii) non-isothermal, at a constant heating rate. Derivative thermogravimetry (DTG) under non-isothermal conditions, namely burning profile, has been applied to obtain reactivity parameters such as the temperature of maximum (peak) combustion rate (PT), burnt out temperature (BT), and activation energy.

Thermal analysis methods (TGDTA) were used to study the effect of combustion improvers on the kinetic parameters of coal combustion.

Coal sample specification

The coal used in this study was brown coal from Novomosvsk coal basin.

EXAMPLE 1

Iron (II) phthalocyanine (0.1-0.2 g) was dissolved in concentrated sulphuric acid (50-60 ml). A sample of brown coal (˜2 g) (2-3 mm grain size) was stirred in this solution for 2 hours at room temperature and left overnight to soak. After stirring, the coal with the deposited phthalocyanine was filtered off. The residual concentration of iron (II) phthalocyanine was determined by UV/visible spectrophotometric analysis. The quantity of the deposited iron-based additive was determined by the difference in concentrations of the starting and residual solutions. The filtered coal was washed with water to neutral pH and air-dried to constant weight over 72-144 hours. Calculations showed that 0.2% of iron (II) phthalocyanine was deposited on the coal. This corresponds to about 200 ppm of iron. After drying, the sample of coal was ground to dust in a mortar for DTA/DTG analysis.

Comparative measurements were made on untreated (‘neat’) brown coal and on brown coal treated under the same conditions as for Example 1 but using concentrated sulphuric acid without dissolved iron phthalocyanine (‘Fe additive’). Results and calculations are graphed in FIGS. 1-6 and discussed below.

The DTA results show much higher exothermal activity in the Fe treated sample compared to untreated brown coal. The effect is particularly pronounced around 100 degrees centigrade, between 350 and 450 degrees centigrade, and between 600 and 800 degrees centigrade. Thermal gravimetric measurement was continued to constant weight, with the treated sample losing 91.2% of its initial weight compared to 86.6% for the untreated coal. Furthermore the treated coal reached constant weight at around 800 degrees centigrade, compared to 850 degrees centigrade for the untreated coal. These results demonstrate the additive of the present invention is surprisingly effective in improving the combustion of solid fuels.

Reaction Model

In treating the obtained DTG data we assumed similarly to the existing literature that the kinetics of coal oxidation is controlled by the first order chemical reaction having the kinetic exponent 0.5<n<1 and that the effect of diffusion can be neglected under the used experimental conditions

dα/dτ=k(1−α)^(n)

where α is the conversion degree, τ is time, k is the temperature dependent Arrhenius rate constant, k=Aexp(−ΔE^(≠)/RT). R is the gas constant, the model parameters A and ΔE^(≠) are the frequency factor and activation energy. The degree of conversion a is give by the expression α=(m_(i)−m_(τ))/(m_(i)−m_(f)), where m_(i) and m_(f) are the initial and final percent masses and m_(τ) the percent mass at time τ as they are recorded during a TG experiment. The real time and temperature are simply related through the constant heating rate T=T₀+βτ. Assuming n=1 straight lines could be obtained plotting in[−in(1−α)T²] vs. 1/T. The value of the activation energy could be deduced from the slope of the straight lines obtained.

The first peak around 100° C. corresponds to the loss of the residual water, the second peak at around 300-400° C. corresponds to the release of the volatile matter. In the third stage a sharp peak is observed due to the char combustion,

The obtained activation energy values were as follows.

Brown coal without additives and untreated with H₂SO₄. ΔE^(≠)=16.8 kJ/mol

Brown coal without additive but treated with H₂SO₄. ΔE^(≠)=16.7 kJ/mol.

Brown coal with Fe additive: ΔE^(≠)=11.3 kJ/mol.

The use of the Fe additive resulted in a decrease in the energy of activation by 5.5 kJ/mol, which is 33% from the initial value of 16.8 kJ/mol. Additive testing on brown coal shows improved carbon burnout, resulting in greater total weight loss:

Carbon content Additive ash reduction Fe Additive 29%

The weight loss was achieved at lower temperature, demonstrating the catalytic action of the additive.

Additive Total Weight Loss None 87.6%/850° C. Fe Additive 91.2%/800° C.

Linear regression data for FIGS. 4-6 are set out below in Tables 1-3.

Linear Regression for neat brown coal (FIG. 4):

TABLE 1 Parameter Value Error A −11.90344 0.06974 B −2020.06766 59.95073 R SD N P −0.98832 0.03655 29 <0.0001

Linear Regression for Brown coal with sulphuric acid (FIG. 5):

TABLE 2 Parameter Value Error A −12.01109 0.04844 B −2008.73036 42.37233

Linear Regression for Brown coal with Fe additive (FIG. 6):

TABLE 3 Parameter Value Error A −12.69625 0.04571 B −1359.01275 40.74655 R SD N P −0.98678 0.02659 32 <0.0001

EXAMPLE 2

As for Example 1 but using cobalt phthalocyanine disulphonate as the metal porphyrin and distilled water instead of sulphuric acid as the fluid carrier.

Results are shown in FIGS. 7-11.

While the present invention has been described with reference to specific examples, it should be understood that modifications and variations of the invention may be constructed without departing from the scope of the invention defined in the following claims. 

1-24. (canceled)
 25. A method for improving the combustion properties of a coal, the method comprising treating said coal with a metal phthalocyanine.
 26. A method according to claim 25, wherein the metal is a transition metal capable of more than one oxidation state.
 27. A method according to claim 25, wherein the metal is selected from the group comprising iron, cobalt, manganese or a mixture of all or any two thereof.
 28. A method according to claim 25, wherein the metal phthalocyanine is iron phthalocyanine.
 29. A method according to claim 25, wherein the coal is brown coal.
 30. A method according to claim 25, wherein the step of treating the coal comprises applying to it a solution of the metal phthalocyanine dissolved in a fluid carrier followed by filtering off the solids and drying them or allowing them to dry.
 31. A method according to claim 30, further comprising the step of washing the coal with water after removal of the solution.
 32. A method according to claim 25, wherein the step of treating the coal comprises vapour-depositing said metal phthalocyanine.
 33. A coal having a metal phthalocyanine deposited thereon.
 34. A coal according to claim 33, wherein the metal is a transition metal capable of more than one oxidation state.
 35. A coal according to claim 33, wherein the metal is selected from the group comprising: iron, cobalt, manganese or a mixture of all or any two thereof.
 36. A coal according to claim 33, wherein the metal phthalocyanine is iron phthalocyanine.
 37. A coal according to claim 33, wherein the coal is brown coal.
 38. A coal according to claim 33, wherein the metal phthalocyanine is present in a concentration in the range 0.05-0.5% by weight.
 39. A method of combustion emission abatement comprising addition of a metal phthalocyanine to coal to form a treated coal, and combusting said treated coal in a combustion chamber with reduction of the excess air in the combustion chamber. 